Lithographic Apparatus Alignment Sensor and Method

ABSTRACT

A lithographic apparatus comprises comprise a substrate table constructed to hold a substrate; and a sensor configured to sense a position of an alignment mark provided onto the substrate held by the substrate table. The sensor comprises a source of radiation configured to illuminate the alignment mark with a radiation beam, a detector configured to detect the radiation beam, having interacted with the alignment mark, as an out of focus optical pattern, and a data processing system. The data processing system is configured to receive image data representing the out of focus optical pattern, and process the image data for determining alignment information, comprising applying a lensless imaging algorithm to the out of focus optical pattern.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of EP application 15183058.5 which wasfiled on 28 Aug. 2015 and which is incorporated herein in its entiretyby reference.

FIELD

The present invention relates to a lithographic apparatus having analignment sensor and to a lithographic alignment method.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In lithography, plural patterns are projected onto a substrate, e.g. inorder to enable manufacturing complex semiconductor structures. Theseplural patterns are projected consecutively onto the substrate. In orderto be able to manufacture patterns at a complexity and small dimensions,a high accuracy of overlay of the patterns is required. In order toreduce so called overlay errors, a plurality of techniques are appliedincluding an alignment of the substrate. In order to align thesubstrate, alignment measurements are performed by an alignment sensor.The alignment sensor essentially measures a position of one of moreknown references that are provided on the substrate, the knownreference(s) e.g. comprising known patterns such as alignment referencepatterns.

In lithography processes, manufacturing cost and its reduction may playa relevant role.

As a result, a designer of substrate (e.g. semiconductor) structuresaims to obtain a large usable area on a substrate, so as to get as manyresulting products from one substrate as possible, thereby to sacrificea part of the surface of the substrate as small as possible. Alignmentmarks are generally placed next to usable areas, i.e. next to (e.g.semiconductor structure) patterns on the surface of the substrate, alsoreferred to as target portions. In order to be able to provide a highalignment accuracy and increase a net yield per substrate, a tendencymay be observed to provide alignment marks in the patterns on thesubstrate, e.g. in a lower layer, whereby successive layers on top ofthe alignment mark provide (e.g. semiconductor) structures. Thus,further layers may be provided on top of the alignment mark, thus usingthe available substrate surface efficiently. Thereby, a usable space ofthe substrate surface is increased, and substrate surface that is usedfor “overhead” purposes only, such as the provision of referencealignment marks, is reduced. Given high overlay requirements, a desirefor a large number of alignment marks to be provided on the substratesurface and distributed over the substrate surface, may come intoexistence. A tendency is observed that the number of layers to beprovided onto the substrate tends to increase, causing the number oflithographic patterns to be successively projected onto the substrate,to increase. Due to the fact that a large number of layers may beprovided onto the substrate, such alignment mark may be hidden by plurallayers provided on top of it during operational use of the lithographicapparatus.

SUMMARY

According to an aspect of the invention, there is provided alithographic apparatus comprising:

a substrate table constructed to hold a substrate; and

a sensor configured to sense a position of an alignment mark providedonto the substrate held by the substrate table,

wherein the sensor comprises

a source of radiation configured to illuminate the alignment mark with aradiation beam,

a detector configured to detect the radiation beam, having interactedwith the alignment mark, as an out of focus optical pattern, and

a data processing system configured to

receive image data representing the out of focus optical pattern, and

process the image data for determining alignment information, comprisingapplying a lensless imaging algorithm to the out of focus opticalpattern.

According to another aspect of the invention, there is provided alithographic alignment method comprising:

providing a substrate having an alignment mark,

emitting a radiation beam onto the alignment mark,

detecting by a detector the radiation beam having interacted with thealignment mark, wherein the radiation beam having interacted with thealignment mark is projected onto the detector as an out of focus opticalpattern,

receiving, from the detector, image data representing the out of focusoptical pattern, and

processing the image data for determining alignment information,comprising applying a lensless imaging algorithm to the out of focusoptical pattern.

According to yet another aspect of the invention, there is provided adata processing system comprising a data input for receiving, from adetector, an out of focus optical pattern, the out of focus opticalpattern from a radiation beam having interacted with an alignment mark,the data processing system being configured to:

receive at the data input image data representing the out of focusoptical pattern, and

process the image data for determining alignment information, comprisingapplying a lensless imaging algorithm to the out of focus opticalpattern.

According to a further aspect of the invention, there is provided acontrol software for being executed by a data processing system, thecontrol software being configured to:

receive image data representing an out of focus optical pattern, the outof focus optical pattern from a radiation beam having interacted with analignment mark and

process the image data for determining alignment information, comprisingapplying a lensless imaging algorithm to the out of focus opticalpattern.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to in which theinvention may be embodied;

FIG. 2 depicts a highly schematic view of an alignment sensor inaccordance with the invention that may be applied in the lithographicapparatus of FIG. 1;

FIG. 3 depicts a highly schematic view of another alignment sensor;

FIG. 4 depicts a highly schematic view of yet another alignment sensor;

FIG. 5A-5B depict a highly schematic view of yet further alignmentsensors

FIGS. 6(a)-6(b) depict a highly schematic view of a still furtheralignment sensor;

FIG. 7 depicts a highly schematic view of a yet still further alignmentsensor; and

FIG. 8 depicts a flow diagram illustrating an operation of the alignmentsensor in accordance with FIGS. 6 and 7.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation or EUV radiation).

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.This technique is known as Optical Proximity Correction (OPC). Opticalproximity correction (OPC) is a photolithography enhancement techniquecommonly used to compensate for image errors due to diffraction orprocess effects. The need for OPC is seen mainly in the making ofsemiconductor devices and is due to the limitations of light to maintainthe edge placement integrity of the original design, after processing,into the etched image on the silicon wafer. These projected imagesappear with irregularities such as line widths that are narrower orwider than designed, these are amenable to compensation by changing thepattern on the photomask used for imaging. Other distortions such asrounded corners are driven by the resolution of the optical imaging tooland are harder to compensate for. Such distortions, if not correctedfor, may significantly alter the electrical properties of what was beingfabricated. Optical Proximity Correction corrects these errors by movingedges or adding extra polygons to the pattern written on the photomask.This may be driven by pre-computed look-up tables based on width andspacing between features (known as rule based OPC) or by using compactmodels to dynamically simulate the final pattern and thereby drive themovement of edges, typically broken into sections, to find the bestsolution, (this is known as model based OPC). The objective is toreproduce, as well as possible, the original layout drawn by thedesigner in the silicon wafer. Generally, the pattern imparted to theradiation beam will correspond to a particular functional layer of adevice such as an integrated circuit being created in the targetportion.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted minorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may, but need not, be of a type having two(dual stage) or more substrate tables (and/or two or more mask tables).In such “multiple stage” machines the additional tables may be used inparallel, or preparatory steps may be carried out on one or more tableswhile one or more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as a-outer anda-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam B, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

-   -   1. In step mode, the mask table MT and the substrate table WT        are kept essentially stationary, while an entire pattern        imparted to the radiation beam is projected onto a target        portion C at one time (i.e. a single static exposure). The        substrate table WT is then shifted in the X and/or Y direction        so that a different target portion C can be exposed. In step        mode, the maximum size of the exposure field limits the size of        the target portion C imaged in a single static exposure.    -   2. In scan mode, the mask table MT and the substrate table WT        are scanned synchronously while a pattern imparted to the        radiation beam is projected onto a target portion C (i.e. a        single dynamic exposure). The velocity and direction of the        substrate table WT relative to the mask table MT may be        determined by the (de-)magnification and image reversal        characteristics of the projection system PS. In scan mode, the        maximum size of the exposure field limits the width (in the        non-scanning direction) of the target portion in a single        dynamic exposure, whereas the length of the scanning motion        determines the height (in the scanning direction) of the target        portion.    -   3. In another mode, the mask table MT is kept essentially        stationary holding a programmable patterning device, and the        substrate table WT is moved or scanned while a pattern imparted        to the radiation beam is projected onto a target portion C. In        this mode, generally a pulsed radiation source is employed and        the programmable patterning device is updated as required after        each movement of the substrate table WT or in between successive        radiation pulses during a scan. This mode of operation can be        readily applied to maskless lithography that utilizes        programmable patterning device, such as a programmable mirror        array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 depicts a highly schematic view of a sensor (also referred to asalignment sensor) according to the invention as may be comprised in thelithographic apparatus of FIG. 1. A substrate table WT holds a substrateW having an alignment mark AM thereon. The alignment sensor comprises analignment source of radiation ASR (also referred to as source ofradiation or radiation source) configured to emit an alignment beam ofradiation AB (also referred to as radiation beam) onto the substrate,and an alignment optical detector AOD (also referred to as detector)configured to detect the alignment beam having interacted with thealignment mark. The alignment sensor is configured to project thealignment beam AB having interacted with the alignment mark AM onto thealignment optical detector AOD as an out of focus optical pattern OP.The optical pattern OP may be considered a spatial distribution of theintensity (and possibly phase) of the radiation that is scatteredtowards the optical detector AOD. The alignment sensor further comprisesa data processing device (or a data processing system) DPD connected tothe alignment optical detector and being configured to receive imagedata representing the out of focus optical pattern as detected by thealignment optical detector, and process the image data to determinealignment information from the received image data. The data processingdevice may either calculate from the received image data a syntheticallyfocused image, and determine alignment information from the calculatedsynthetically focused image, or may directly determine the alignmentinformation from the received image data. In the latter case, theintermediate step of calculating the synthetically focused image fromthe received image data may be omitted and the alignment informationbeing derived from the received image data.

The alignment sensor is applied to derive a position of an alignmentmark. The alignment mark is provided on the substrate (also referred toas a wafer). It is noted that in this document the terms “substrate” and“wafer” are used interchangeably. The alignment mark may form analignment grating or any other suitable alignment mark. A position ofthe alignment mark as sensed may be compared to a reference position ora position of the same alignment mark at a previous step in the waferprocessing, and alignment of the wafer may be performed based on themeasurement obtained from the alignment sensor. The alignment beam maybe a laser beam or any other suitable optical beam such as amonochromatic beam (i.e., the radiation having substantially a singlewavelength), a beam having a wavelength within a range of wavelengths,etc. The beam, having interacted with the alignment mark, e.g. by meansof diffraction, scattering or any other suitable interaction, isdetected by the alignment optical detector, such as an optical detectorarray, e.g. a CCD or CMOS optical detector array or any other suitableoptical detector for detecting a spatial distribution of e.g., intensityor color, within the capturing area of the alignment optical detector.

In accordance with an embodiment of the invention, as described abovewith reference to FIG. 2, the alignment sensor projects the beam out offocus onto the alignment optical detector. The expression “out of focus”is to be understood as lacking focus to an extent that the alignmentpattern is not sharply visible in the image itself as received by thealignment optical detector. In order to provide derive usable data fromthe out of focus optical pattern, a so called lensless imaging techniqueis applied. Attributes of incoming electromagnetic radiation (e.g.,intensity, phase) of the out of focus optical image are detected andtransformed into digital data that is thereupon subjected to operationsin order to extract information from the radiation. That is, thelensless imaging may emulate in software an optical system. As theoptical system is emulated in software, faults or aberrations as occurin an optical system using lenses, focusing mirrors, and/or otheroptical elements, may be omitted. Hence, in principle, the lenslessoptical imaging may emulate a theoretically ideal optics for acting onradiation of any wavelength.

The alignment sensor may make use of so called lensless optics. It isnoted that the expression “lensless optics” may (however does notnecessarily need to) refer to an optical imaging system that does notcomprise any refractive or reflective optical component (such as a lens,a focusing mirror, etc.).

Using the alignment sensor in accordance with the invention, a largewavelength range may be covered. For example, an alignment beamwavelength range from 0.1 nanometers to 1500 nanometers may beenvisaged. As the processing of the synthetic optical image in softwaredoes not or to a lesser degree suffer from wavelength-dependentaberrations, an absence or partial absence of such optical components(such as a transmissive lens, a focusing mirror, a polarizer, etc), alarge wavelength range may be applied. Using such a large wavelengthrange, “hidden” alignment patterns, i.e. alignment patterns that arecovered by further layers provided in the lithographic process, may bedetected more easily, as it increases the likelihood that the furtherlayers are transparent to at least some wavelengths occurring in thelarge wavelength range. The layers provided on top of the alignment markmay differ substantially in their optical transmission characteristic.For example, a layer comprising a film of metal may have differentoptical transmission characteristics for a specific wavelength than alayer of oxide. In the alignment sensor as described a large wavelengthrange may be applied. A likelihood that the layer or layers on top ofthe alignment pattern show a transparency for specific wavelengths mayincrease when using an alignment beam of a large wavelength range. Thatis, the chances that a particular wavelength is found that provides asufficiently high transmission may increase when using the largewavelength range.

Using the lensless imaging algorithm, the optical imaging on thedetector need only be such that the relevant attributes of the incidentradiation can be captured that enable to convert the radiation into datafor digitally emulating the theoretically ideal optical system.Accordingly, the detector AOD in FIG. 2 represents the interface betweenthe physical domain and the optical domain. There may, but need not be,one or more optical elements in the radiation path from the mark AM tothe detector.

The data processing device DPD may be formed by a separate dataprocessing device, such as a microprocessor provided with suitablesoftware instructions, or may be integrated into other data processingdevices of the lithographic apparatus. In other words, the tasksperformed by the data processing device may for example be implementedas tasks (processes) running on an existing data processing device ofthe lithographic apparatus. The alignment beam may be any suitablealignment beam, such as an (optical) laser beam. The alignment source ofradiation may correspondingly be any suitable source of radiation, suchas a laser. As the present development enables to operate the alignmentbeam over a wide wavelength range, e.g. ranging from UV to IR, acorresponding alignment source of radiation (providing a wide spectrumor having an adjustable wavelength) may be applied. The alignment markmay be any suitable alignment mark. For example the alignment marks maycomprise diffractive alignment marks. Any other suitable alignment markmay be applied.

A variety of techniques have been devised that may be applied to obtainphase information, and/or to calculate the synthetically focused image,from the image as detected by the alignment optical detector, using thelensless imaging algorithm. Some examples are described below withreference to FIGS. 3, 4, 5A and 5B. It is noted that, although in eachof those examples, a synthetic image is calculated, and the alignmentinformation derived from the synthetic image, the step of calculatingthe synthetic image may be omitted, i.e. the alignment information maybe directly derived instead of the synthetic image, using the lenslessimaging algorithm.

In an embodiment, as schematically depicted in FIG. 3, the alignmentsource of radiation ASR comprises a wavelength selective element WSEconfigured to receive a wavelength parameter WP and to control awavelength range of the alignment beam in response to the wavelengthparameter WP. The data processing device DPD is then configured tocalculate the synthetic image from the wavelength parameter and from thereceived image data for each wavelength parameter, i.e. from thereceived image data as a function of the wavelength parameter. As theoptical image as received by the alignment optical detector will show adependency on wavelength, this dependency may be used to derive phaseinformation and thereby to calculate the synthetically focused image.For example, a change in the wavelength may translate into a change inthe optical pattern as detected by the alignment optical detector, asthe changing wavelength—at unchanged optical paths—results in a changeof mutual phases between interfering beam parts. The wavelengthparameter WP may be provided by any suitable parameter, such as ananalogue or digital signal representative of a numerical value of thewavelength in nanometers. The wavelength selective element may be anysuitable optical element, such as a controllable filter, e.g. acontrollable (e.g. tunable) band pass optical filter or a controllable(tunable) narrowband optical filter.

In an embodiment, as schematically depicted in FIG. 4, the lithographicapparatus is configured to move the substrate table and the alignmentoptical detector relative to each other so as to alter an opticaldistance OD between the substrate W held by the substrate table and thealignment optical detector AOD, the data processing device beingconfigured to calculate the synthetic image from the optical distanceand from the received image data for each optical distance, i.e. fromthe received image data as a function of the optical distance.Accordingly, by altering the optical distance, the unfocussed image onthe alignment optical detector changes, which is used to derive phaseinformation. The phase information may be obtained from the change ofthe image as detected by the alignment optical detector in relation tothe change in the optical distance. This technique may be applied with amonochromatic beam as well as with a wideband beam. Displacing thesubstrate by moving the substrate table in a direction perpendicular tothe surface of the substrate (generally a vertical direction) may beperformed by substrate table actuators WTA, which may for example beformed by the substrate table positioner WP (as depicted in FIG. 1), sothat no additional hardware in terms of displacement actuators may berequired. Alternatively, or in addition to the displacement of thesubstrate table in vertical direction, the substrate table may also bemoved in horizontal direction in order to alter the optical distance.

In an embodiment, the data processing device is configured to calculatethe synthetic image using an iterative reconstruction algorithm. Basedon an initial assumption, an initial synthetic image may be calculated.Using a step by step approach, the calculated image may iterate towardsthe synthetic image. The calculation of the synthetic image using aniterative reconstruction algorithm works as follows. One collects a setof constraints that the pattern must fulfill, by means of measurements,or by means of prior knowledge (such as when one knows what pattern thealignment mark will have). The reconstruction solution will meet allthese constraints. Hence one can think of this reconstruction problem asa feasibility problem, in which a possible solution that satisfies allthese constraints is identified. Identifying a solution to thisfeasibility problem can be done by means of a alternating projectionalgorithm, in which the candidate solution is projected (orthogonally ina high dimensional space) onto each constraint one by one. When thecandidate solution satisfies all constraints (sufficiently) one canterminate the algorithm.

In an embodiment, as schematically depicted in FIG. 5A, the alignmentsensor further comprises an alignment beam reference path ABRPconfigured to provide an alignment reference beam ARB, the alignmentreference beam to interact with the alignment beam AB at the alignmentoptical detector AOD, the data processing device DPD being configured tocalculate the synthetic image from the received image data, the receivedimage data resulting from the interaction of the alignment beam and thealignment reference beam. Accordingly, as the interaction between thealignment reference beam and the alignment beam at the alignment opticaldetector depends on a phase of the alignment beam at the alignmentoptical detector, phase information may be derived therefrom. Thesynthetically focused optical image may be calculated therefrom,examples being provided below. In the first example an optical lengthchanges of the reference path, along which the reference beampropagates. In the second example a wavelength of the reference beamchanges.

In the first example, as schematically depicted in FIG. 5A, thealignment beam reference path ABRP comprises a movable referencestructure MRS. The alignment sensor is configured to move the referencestructure MRS so as to alter an optical length of a propagation path ofthe reference beam ARB. The data processing device DPD is configured tocalculate the synthetic image from the optical length and from thereceived image data for each optical length. The phase information maybe derived from the changes in the optical signal detected by thealignment beam optical detector as a result of the changes in theoptical length of a propagation path of the reference beam ARB, and thusof the phase of the reference beam incident on the alignment opticaldetector (as, given a varying optical path length, the phase at thealignment optical detector will change as a function of the optical pathlength).

In the second example, as schematically depicted in FIG. 5B, thealignment source of radiation ASR comprises a wavelength selectiveelement WSE configured to receive a wavelength parameter WP and tocontrol a wavelength range of the alignment beam in response to thewavelength parameter WP. The data processing device DPD is configured tocalculate the synthetic image from the wavelength parameter and from thereceived image data for each wavelength parameter, i.e. from thereceived image data as a function of the wavelength parameter. The phaseinformation may be derived from the changes in the optical signaldetected by the alignment beam optical detector as a result of thechanges in the wavelength of the reference beam ARB, and thus of thephase of the reference beam incident on the alignment optical detector(as, given a fixed optical path length, the phase at the alignmentoptical detector will change as a function of the wavelength).

In an embodiment, the data processing device is configured to correlatethe synthetic image with an expected image representing the alignmentmark and to derive alignment information from a result of thecorrelation. The expected image represents an image that would have beenexpected, knowing a shape, optical characteristics and approximateposition of the alignment mark on the substrate. A correlation may bedetermined for various possible positions of the alignment mark on thesubstrate, whereby the highest correlation may provide information aboutthe one of the possible positions that would most closely correspond toa position of the alignment mark. This process may be performed in aniterative way thereby to increase accuracy of determination of aposition of the alignment mark on the substrate. This correction also beperformed with an only partly synthetically focused, i.e. not perfectlyfocused synthetic image. Additional filtering may for example be appliedthereto. In case of a partly (optically) focused image, it can berefocused digitally if the phase has been retrieved. If this cannot bedone, the correlation peak is not as strong but the alignment can stillbe done (with a lower accuracy). It is noted that, alternatively,instead of the iterative reconstruction, a so called “matched filter”algorithm may be applied.

In an embodiment, the alignment sensor does not comprise a focusingoptical element (such as a lens or a mirror) in an optical path from thealignment mark to the alignment optical detector.

A possible embodiment using the computational optics is described belowin more detail with reference to FIGS. 6-8.

FIG. 6 shows schematically an alignment sensor for performing alignmentmeasurement. A modified form of “lensless imaging” or coherentdiffractive imaging (CDI) is used. CDI, which is related also to digitalholography, is a technique that has been proposed for use in microscopy.In the present disclosure, the CDI technique is adapted to performalignment on diffractive structures, for example measurement ofasymmetry of grating structures. The alignment sensor, while notnecessarily being completely lensless, avoids the need for the verycomplicated high-NA, wideband objective lens and other optical elements,required to meet performance requirements in future applications.

The alignment sensor of FIG. 6(a) comprises a radiation source 611 andan image sensor 623. Radiation source 611 in this example supplies abeam 630 of spatially coherent radiation. Source 611 may be formed byone or more narrowband (monochromatic) laser sources, in which case theradiation will be both spatially coherent and temporally coherent.Alternatively, and assumed in the present example, source 611 may be abroadband source which is spatially coherent, with a low temporalcoherence. Such a source may be a so-called supercontinuum source or“white light laser”. Source 611 may be supplemented with other devicesin an illumination system 612 to deliver the beam 630 in a desired form.For example the source 611 and illumination system in some embodimentsmay include a wavelength selector 613 (shown dotted). Such a wavelengthselector may be for example, an acousto-optic tunable filter (AOTF).

Image sensor 623 can be a CCD or CMOS sensor. Different illuminationmodes can be implemented by providing an aperture device, a programmablespatial light modulator, or spatially distributed fibers.

In an illumination path from source 611 to target T, an illuminationoptical system comprises a simple mirror 640 and low-NA lens 642. Lens642 focuses illuminating radiation beam 630 into a spot S at thelocation of alignment target T on substrate W. A positioning system(similar for example to the positioning system PW in the lithographicapparatus LA) brings the substrate W and target T to the focal point ofbeam 630. The spot may have a similar size and shape of for exampleroughly a circle of diameter in the range 10 to 80 μm, for example 20 to50 μm or around 40 μm. In an embodiment where the illuminating radiationbeam 630 is incident at an oblique angle as shown, the spot S may benon-circular, or anamorphic optics can be applied to achieve a circularspot. Radiation 646 reflected by the target (diffracted at zero order)is illustrated for simplicity as being dumped at 648. In a practicalembodiment, the reflected (zero order) radiation can be used, forexample to determine the focus position of the substrate as part of aposition control mechanism. Radiation 650 comprising a desired portionof the radiation scattered by the target T is collected by sensor 623.No high-NA objective lens is required in order to collect the objectradiation, and the radiation can pass directly from target to sensor. Ina practical example, a simple collection optical system may be provided,for at least roughly collimating the beam (reducing divergence). Such acollection optical system, which may be a simple lens, is shownschematically in the inset diagram at (b). Nevertheless, the complexhigh-NA objective lens is eliminated. The illuminating radiation can bedirected directly at the target area, bypassing the collection opticalsystem. This helps to avoid noise caused by scattering of theilluminating radiation within elements of the optical system.

In addition to the collected scattered radiation 650, referenceradiation 652 is also delivered to the sensor 623. The scatteredradiation 650 and reference radiation 652 are derived from the samesource 611 so as to be coherent with one another and consequently forman interference pattern at the sensor, depending on their relativephases at each pixel on the sensor. In the illustrated example,reference radiation 652 is obtained by splitting off a portion of theilluminating radiation 630 with a beam splitter 654 and delivering it tothe sensor via a movable mirror 656, a diverging lens 658 and a foldingmirror 660. The reference radiation 650 floods the image sensor 623 witha “reference wave” having a relatively uniform amplitude across thefield of sensor 623. The reference wave travels in a direction obliqueto an optical axis of the system at a well-defined angle, and so thereference wave has a well-defined amplitude and phase. The scatteredradiation 650, which may be referred to as the object wave, has unknownamplitude and phase.

As an alternative to splitting off a portion of the illuminatingradiation to form a reference wave, so-called “self-referencing”arrangements are also possible. In that case, a portion the higher orderscattered field itself is split off and used as a reference wave. Forinstance, a self-referencing arrangement may work by interfering asheared copy of the scattered field with the scattered field.

As will be explained further below, interference between the referencewave and the object wave gives a resulting intensity distribution on thesensor 623 that can be used by processor PU to calculate the complexradiation field of the scattered object wave (“complex” here meaningboth amplitude and phase). Image data 662 is delivered to processor PU,representing one or more of these captured intensity distributions. Wavepropagation algorithms can then be used to calculate a synthetic image,without the need for imaging optics.

Is it not essential that the reference wave is at an oblique angle.However, by using an oblique angle one can introduce a fringe patternacross the target that has a high spatial frequency and can be used to“determine” the phase information from a single image acquisition. Theangle of the reference wave must not be too large, for example less thanthe wavelength divided by twice the pixel array pitch (lambda/2*pixelsize). In a typical set-up, 3-4 degrees may be sufficient, for example.Without this high-frequency fringe pattern, one can obtain the phaseinformation for example by “phase-stepping”. As described below, onemethod for phase stepping is where one acquires multiple images whilevarying the relative phase of the reference beam. While this can bedone, it puts rather severe demands on the stability of the set-up, andthe oblique reference beam can therefore be advantageous. In othermethods, phase stepping can be done by spatial modulation, such thatdifferent phase steps are found within a so-called ‘super-pixel’. Theterm super-pixel may be understood as a collection of pixels, forexample based on their neighborhood, or phase, or amplitude, orintensity, or correlation, etc.

The sensor placement and the pitch of its array of pixels should bedetermined such that the pixel array provides adequate sampling of theinterference pattern. As a rough guide, the pixel spacing (pitch) shouldbe less than for example λ/2d, where λ, is the (longest) wavelength inthe illuminating radiation 630 and d is the spacing from target T toimage sensor 623. In a practical example, the spacing d may be of theorder of 1 cm. The sensor dimension may be several times d in eachdirection (X and Y), for example five or more times d, ten times d oreven larger. In this regard, it will be noted that the drawings of FIGS.6 and 7 are very much distorted in scale, to allow a clear depiction ofthe optical system. The sensor in practice may be very much closer tothe target, or very much wider in extent, than the drawings suggest. Forexample, the sensor may have a distance d and extent L such that itsubtends a relatively wide angle θ when seen from the target T. Theangle θ may be over 100 degrees in each dimension, for example over 135degrees, and for example around 150 degrees. As illustrated in the insetFIG. 6(b), a simple collimating lens 664 can be used to increase thephysical distance to the sensor, while still capturing a large range ofangles of scattered radiation. The extent of the sensor does not need tobe centered over the target as shown. It merely needs to be positionedto capture the desired diffraction orders, based on the angle ofincidence of the illuminating radiation, the wavelength(s) of theilluminating radiation and the pitch of the periodic grating.

Variations are possible, for example in the delivery of the referencewave. In the illustrated example, the movable mirror 656 can be used forpath length compensation, adjusting the optical path difference betweenthe object wave and the reference wave. If the source 611 is a broadbandsource such as a white light laser, then stepping with the mirror allowsa spectroscopic measurement of the complex radiation field over a largewavelength range. Since the coherence length of a broadband source isrelatively small, the apparatus may operate by capturing images whilestepping through a wide range of positions. Only some of thosepositions, corresponding to path length differences close to zero, willbe within the coherence length. Other positions will not yield a complexfield image. Note that the path length difference may be different atdifferent locations on the sensor, at a given position of the mirror656. Consequently, each point in the sampled far field will have amaximum fringe contrast at a different position of the mirror. In orderto calculate the phase and/or amplitude for a specific wavelength, onecould still need to include the information from multiple images in thecalculations. In case of low-coherence sources, one will get a contrastvariation across the image. This could be calibrated with a testmeasurement on a test target.

In addition to relaxing design challenges for a given size of imagefield, the elimination of the complex objective lens allows a largerfields of view to be implemented that would simply be impossible withconventional optics. Instead of a 2×2 array of gratings, for example, acomposite target could be imaged with 5×2 or even 5×4 gratings withinthe field of view.

In order to obtain unambiguous complex radiation field information, thesteps of movable mirror 656 can be made much smaller than a (longest)wavelength of the illuminating radiation. In measurement of targets inhigh-volume manufacturing examples such as semiconductor devicemanufacturing, the time taken per measurement is critical, but alsoincludes not only the time take for image capture itself but also thetime for moving and acquiring each target, prior to image capture. Oncethe target is acquired, to capture multiple images while stepping themoving mirror 656 may not add significantly to the overall alignmentmeasurement time. The number of steps taken may therefore be quite largein practice, even if many of the captured images contribute little ornothing in the subsequent analysis. Also, if a larger field of view isobtained, more individual gratings or other target structures can bemeasured in one capture operation.

In other examples, phase stepping can be realized without moving partssuch as movable mirror 656. For example, a reflective or transmissivespatial light modulator could be provided with different phase steps atdifferent pixel positions within larger ‘superpixels’. Different phasesteps could be implemented by etching steps into a suitable material, orby more exotic means. A spatial light modulator based on, for example,liquid crystal, could be used to modulate the phase. In other examples,the wavelength of the reference wave can be varied instead of or inaddition to its path length. Provided the wavelength and incidence angleis known, the complex radiation field can be calculated. Wavelengthselection can be made by inserting filters in the illumination path,and/or by selecting different radiation sources, or tuning a tunablesource.

In other words, phase information can be obtained by varying path lengthdifference with a constant wavelength, by varying wavelength with aconstant path length difference, or by a combination of both variations.Wavelength selection can be applied after scattering, if desired. Forexample wavelength-selecting filters can be inserted in front of imagesensor 623 and changed between captures. Multiple image sensors 623could be provided, with the collection path being split bywavelength-selecting beam splitters. Different pixels within the sameimage sensor 623 can be made sensitive to different wavelengths, forexample in the manner of RGB filter arrays on a single-chip color imagesensor.

FIG. 7 shows another variation. Most parts are similar to thoseillustrated in FIG. 6 and the same reference signs are used. The maindifference is that the reference radiation 652 is not taken directlyfrom the illuminating radiation 630 but is taken by mirror 670 from thezero order radiation 646 reflected by target T. This variation may ormay not simplify the optical layout. A benefit of this variation is thatthe scattered radiation 650 (object wave) and the reference radiation652 (reference wave) will be subject to the same influences over agreater portion of their respective optical paths. In particular, anyvibrations experience by the target relative to the optical system willinfluence both the reference wave and the object wave substantiallyequally. Therefore the influence of these vibrations on the recordedcomplex field will be reduced. The reference wave in this arrangementwill carry some information about the target structure, but this will beonly average information, and the reference wave is still effective as aphase reference for the purpose of measuring the complex radiation fieldof the object wave.

FIG. 8 illustrates the complete measurement process using the apparatusof FIG. 6 or 7. The process is implemented by operation of the opticalhardware illustrated in the drawings, in conjunction with processor PU.An example using the lensless imaging algorithm is described. Functionsof (i) controlling the operations of the hardware and (ii) processingthe image data 662 may be performed in the same processor, or may bedivided between different dedicated processors. Processing of the imagedata need not even be performed in the same apparatus or even in thesame country.

At 802 a, 802 b, . . . 802 n a set of intensity distribution images arecaptured and received by processor PU from the image sensor 623. Alsoreceived is auxiliary data (metadata) 804 defining operating parametersof the apparatus associated with each image, for example theillumination mode, position of mirror 656 and the like. This metadatamay be received with each image, or defined and stored in advance forthe set of images. The metadata may also include information of thesubstrate and target structure. Also received or previously stored isreference wave specification 806 defining the known phase of thereference wave as it varies across image sensor 623. The absolute phasedoes not need to be known as long as you accurately know the relativephase steps across the image sensor, and/or relative to an arbitraryinitial position of the movable mirror. Additional calibrationprocedures can be provided to obtain this information, rather thanrelying on design and calculation alone.

From the received image data 802 a etc., the metadata 804 and thereference wave specification 806, processor PU calculates a complexradiation field 810. This is a representation of amplitude and phase ofthe object wave (scattered radiation 650) across the image sensor 623.The representation may be expressed in the form of amplitude and phasevalues per pixel position. Other equivalent forms of expression are alsoavailable. From this complex radiation field, processor PU in a process812 can use wave propagation algorithms to calculate a synthetic image814 as it would be seen if focused by an ideal optical system onto animage sensor (similar to sensor 623 of FIG. 6).

As illustrated schematically in FIG. 8, synthetic image 814 can have thesame form as the real image. Dark and light rectangles corresponding toeach individual grating in a composite target are shown, just by way ofexample. The synthetic image may be an image of intensity, analogous tothe real images captured in the known apparatus. However, the syntheticimage does not necessarily have to be an intensity image. It can also bea phase image of the grating, or both intensity and/or amplitude andphase images can be calculated. As already discussed above, two suchimages can be used to calculate asymmetry of each grating, if the twoimages are produced using opposite portions of a diffraction spectrum ofthe target. In FIG. 8 a second synthetic image 814′ is shown. It will beunderstood that the second synthetic image is obtained by the sameprocess as image 814, based on a second set of images 802 a′, etc. thatare captured using image sensor 623 when the illumination profile ororientation of the target has been rotated 180 degrees. In other words,the synthetic image 814 is be produced using (for example) +1 orderdiffracted radiation, while synthetic image 814′ is produced using −1order diffracted radiation.

At step 820 processor PU compares intensities of the images of differentgratings in the images 814 and 814′ to obtain asymmetry measurements ofeach grating. At step 822 the measured asymmetries of the multiplegratings within composite target are converted by a predeterminedformula and/or calibration curves to obtain a measurement of a parameterof interest, such as overlay OV, focus F or dose D. The formulae arederived from knowledge of the target structures, including the biasscheme applied. Calibration curves may be obtained by comparingasymmetry measurements on a range of targets with measurements of theparameter of interest made by other techniques such as electronmicroscopy (SEM, TEM).

The illustrated process is repeated for all targets of interest. Notethat the computational parts of the process can be separated in time andspace from the image capture. The computations do not need to becompleted in real time, although of course that would be desirable. Onlythe capturing of the images need 802 a etc. requires the presence of thesubstrate, and so impacts productivity throughput) of the lithographicdevice manufacturing process overall.

As mentioned above, the number of images 802 a captured may be greaterthan the number selected and used to calculate the complex radiationfield. The number used can be selected according to requirements. Inprinciple, four images captured with different (known) phase stepsbetween the object wave and the reference wave should be sufficient toobtain unambiguous amplitude and phase information. Alternatively, fourimages captured with different (known) wavelengths of illuminatingradiation 630 would be sufficient. Greater numbers can be used toimprove measurement certainty. The number of images required for thecalculation may be reduced if the calculation can be constrained usingknowledge of the target structure and substrate. Phase steppingalgorithms are known which are proven to be more robust to noise. Forexample five-step phase shifting algorithms are more robust to phaseshifter calibrations. Multi-step algorithms exist which do not requirethe knowledge of the phase step as long it is identical. Random phasestep algorithms also exist. See for example, James C Wyant, “PhaseShifting Interferometry.nb.pdf', Optics 513 Chapter 5, Chapter Notes,2011 available at_http://fp.optics.arizona.edu/jcwyant/Optics513/ChapterNotes/Chapter05/Notes/Phase%20Shifting%20Interferometry.nb.pdf.

In the above examples the steps of calculating the complex field 810 andcalculating the synthetic image 814 are shown as separated sequentially.This is likely to be a convenient way to proceed in practice. Inprinciple, however, the calculations could be merged so that oneproceeds by a single calculation directly from the captured images 802 aetc. to the synthetic image 814, without explicitly calculating thecomplex field. The claims should not be interpreted as requiringexplicit calculation of the complex field as a distinct array of data.

In addition to calculating synthetic images of the target as it would beseen by image sensor 623 of the known dark-field imaging scatterometer,the apparatus can calculate synthetic images of the diffraction patternas it would be seen in a pupil image sensor. Unlike the known apparatus,no splitting of the collected radiation into different optical branchesis required to obtain these different images.

The lensless imaging algorithm and the deriving of the alignmentinformation may employ software or other suitable programming such asthe programming of a signal processor, the programming of a gate array,etc. The software may be executed on any suitable data processor, suchas a microcontroller, microprocessor, digital signal processor, etc.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus comprising: a substrate table constructed tohold a substrate; and a sensor configured to sense a position of analignment mark provided onto the substrate held by the substrate table,wherein the sensor comprises: a source of radiation configured toilluminate the alignment mark with a radiation beam, a detectorconfigured to detect the radiation beam, having interacted with thealignment mark, as an out of focus optical pattern, and a dataprocessing system configured to: receive image data representing the outof focus optical pattern, and convert the received image data into datafor digitally emulating an optical system to determine alignmentinformation.
 2. The lithographic apparatus of claim 1, wherein: thesensor comprises an optical propagation path configured for propagatingthe radiation beam, having interacted with the alignment mark, to thedetector, and the optical propagation path is a lensless opticalpropagation path.
 3. The lithographic apparatus of claim 1, wherein: thesensor comprises an optical propagation path configured for propagatingthe radiation beam, having interacted with the alignment mark, to thedetector, and the optical propagation path is a non-focusing opticalpropagation path.
 4. The lithographic apparatus of claim 1, wherein thedata processing system is configured to convert the received image datainto data for digitally emulating an optical system to determinealignment information by being configured to: calculate, from thereceived image data, a synthetically focused image; and determine thealignment information from the calculated synthetically focused image.5. The lithographic apparatus of claim 1, wherein the source ofradiation comprises: a wavelength selective element configured toreceive a wavelength parameter and to control a wavelength range of theradiation beam in response to the wavelength parameter, the dataprocessing system being configured to calculate the syntheticallyfocused image from the received image data as a function of thewavelength parameter.
 6. The lithographic apparatus of claim 1, wherein:the lithographic apparatus is configured to move the substrate table andthe detector in respect of each other so as to alter an optical distancebetween the substrate held by the substrate table and the detector, andthe data processing system being configured to determine the alignmentinformation from the received image data as a function of the opticaldistance.
 7. The lithographic apparatus of claim 1, wherein the dataprocessing system is configured to determine the alignment informationusing an iterative reconstruction algorithm.
 8. The lithographicapparatus of claim 1, wherein: the sensor further comprises an radiationbeam reference path configured to provide a reference beam to interactwith the radiation beam at the detector, the data processing systembeing configured to determine the alignment information from thereceived image data, and the received image data resulting from theinteraction of the radiation beam and the reference beam.
 9. Thelithographic apparatus according to claim 8, wherein: the radiation beamreference path comprises a movable reference structure, the sensor beingconfigured to move the reference structure so as to alter an opticallength of a propagation path of the reference beam, and the dataprocessing system being configured to determine the alignmentinformation from the received image data as a function of the opticallength.
 10. The lithographic apparatus of claim 8, wherein: the sourceof radiation comprises a wavelength selective element configured toreceive a wavelength parameter and to control a wavelength range of theradiation beam in response to the wavelength parameter, and the dataprocessing system being configured to determine the alignmentinformation from the received image data as a function of the wavelengthparameter.
 11. The lithographic apparatus of claim 6, wherein the dataprocessing system is configured to determine the alignment informationfrom the received image data by determining the synthetic image from thereceived image data and to determine the alignment information from thesynthetic image.
 12. The lithographic apparatus of claim 1, wherein thedata processing system is configured to correlate and/or filter thesynthetic image with an expected image representing the alignment markand to derive alignment information from a result of the correlationand/or filtering.
 13. The lithographic apparatus of claim 1, wherein thesensor does not comprise a focusing optical element in an optical pathfrom the alignment mark to the detector.
 14. A lithographic alignmentmethod comprising: emitting a radiation beam onto an alignment mark on asubstrate, detecting by a detector the radiation beam having interactedwith the alignment mark, wherein the radiation beam having interactedwith the alignment mark is projected onto the detector as an out offocus optical pattern, receiving, from the detector, image datarepresenting the out of focus optical pattern, and converting thereceived image data for digitally emulating an optical system todetermine alignment information. 15.-17. (canceled)
 18. A dataprocessing system comprising: a data input configured to receive, from adetector, an out of focus optical pattern, the out of focus opticalpattern from a radiation beam having interacted with an alignment mark,wherein the data processing system being configured to: receive at thedata input image data representing the out of focus optical pattern, andconvert the received image data into data for digitally emulating anoptical system to determine alignment information.
 19. (canceled) 20.The lithographic apparatus of claim 1, wherein the source of radiationcomprises one or more spatially coherent radiation sources.
 21. Thelithographic apparatus of claim 1, wherein the source of radiationcomprises a supercontinuum source.
 22. The lithographic apparatus ofclaim 1, wherein the source of radiation comprises a tunable radiationsource.